#include <iostream>
#include <algorithm>
#include <vector>
#include <utility>
#define llint long long
#define inf 1e18
#define mod 1000000007

using namespace std;
typedef pair<llint, llint> P;

llint n;
llint t[3005], x[3005];
vector<P> vec;
llint dp[3005][3005];
const int FACT_MAX = 200005;
llint fact[FACT_MAX], fact_inv[FACT_MAX];

llint modpow(llint a, llint n)
{
	if(n == 0) return 1;
	if(n % 2){
		return ((a%mod) * (modpow(a, n-1)%mod)) % mod;
	}
	else{
		return modpow((a*a)%mod, n/2) % mod;
	}
}

void make_fact()
{
	llint val = 1;
	fact[0] = 1;
	for(int i = 1; i < FACT_MAX; i++){
		val *= i;
		val %= mod;
		fact[i] = val;
	}
	fact_inv[FACT_MAX-1] = modpow(fact[FACT_MAX-1], mod-2);
	for(int i = FACT_MAX-2; i >= 0; i--){
		fact_inv[i] = fact_inv[i+1] * (i+1) % mod;
	}
}

llint comb(llint n, llint k)
{
	llint ret = 1;
	ret *= fact[n];
	ret *= fact_inv[k], ret %= mod;
	ret *= fact_inv[n-k], ret %= mod;
	return ret;
}

int main(void)
{
	ios::sync_with_stdio(0);
	cin.tie(0);
	make_fact();
	
	cin >> n;
	for(int i = 1; i <= n; i++){
		cin >> t[i] >> x[i];
		vec.push_back(P(x[i], t[i]));
		if(t[i] == 1) vec.back().first--;
	}
	sort(vec.begin(), vec.end());
	
	llint cnt = 0;
	dp[0][0] = 1;
	for(int i = 0; i < n; i++){
		llint x = vec[i].first, t = vec[i].second;
		//cout << x << " " << t << endl;
		for(int j = 0; j <= n; j++){
			if(t == 0){
				if(x-(cnt+j) >= 0){
					dp[i+1][j] += dp[i][j] * (x-(cnt+j)) % mod;
					dp[i+1][j] %= mod;
				}
			}
			else{
				dp[i+1][j] += dp[i][j], dp[i+1][j] %= mod;
				if(j+1 <= n && x-(cnt+j) >= 0){
					dp[i+1][j+1] += dp[i][j] * (x-(cnt+j)) % mod;
					dp[i+1][j+1] %= mod;
				}
			}
		}
		if(t == 0) cnt++;
	}
	
	/*for(int i = 0; i <= n; i++){
		for(int j = 0; j <= n; j++){
			cout << dp[i][j] << " ";
		}
		cout << endl;
	}*/
	
	llint ans = 0;
	for(int i = 0; i <= n; i++){
		llint tmp = dp[n][i];
		tmp *= fact[n-(cnt+i)], tmp %= mod;
		if(i % 2) ans += mod - tmp, ans %= mod;
		else ans += tmp, ans %= mod;
	}
	cout << ans << endl;
	
	return 0;
}